3°) flash this downgrade emu to sysnand <---- not sure if this works or bricks the 3ds

4°) then system transfer using spoof fw

2nd theory

1°) transfer blue 03ds 9.7 to pink 9.7 emunand

2°) import all user settings from this 9.7 emunand to my previous 9.2 backup <---dont know if RXtools can do all of that, except the ticket.db i don't know the use of the others files we can mess with (moveable.sed etc...)

3°) flash this transform 9.2 nand to sysnand

4°) then system transfer using spoof fw

so if you guys could teach me on the obscure parts I don't really know

I'm in your same situation.
On rx emunand the spider browser exploit works and maybe it's possible to skip the flash emunand to sysnand, but I'm stuck on 9.7 rx emunad and I doesn't know how to downgrade

Give a try at downgrading your 9.7 RX EmuNAND before doing the system transfer, backup with EmuNANDTool, and use FBI to remove and install each CIA files from the 9.0 or 9.2 update (you can find the list @3DBrew, and I even think there are already packaged zip on "That Other Site"...), be sure the test if web browser is exploitable ! That downgrade operation work with Gateway's EmuNAND 9.5 to prior version so it should be OK.
If it worked, restore 9.7 RX EmuNAND, do the System Transfer between the O3DS, then downgrade the RX EmuNAND to 9.0 or 9.2, run browser exploit to spoof latest version on O3DS EmuNAND, boot NTR 2.2 debugger on 9.0 N3DS and send the magic command, initiate a System Transfer and it will be good